This artificial cave looks like something out of a fairytale.

Around 100,000 bodies are buried below the medieval headstones, many of which mark graves with multiple bodies stacked up to 12 deep.

An underground city of salt.

The former Nazi Germany concentration camp stands as a museum to remember this dark chapter in European history.

This overlooked courtyard and museum celebrate Benjamin Franklin's life and achievements.

One of the world's oldest continually operating bookstores.

This extraordinary Depression-era expenditure was modeled after a postcard of a Mediterranean hotel.

This jail which was in service for over a century is now open for curious visitors.

The internal railway of one of the biggest steel plants in America has been transformed into a linear park with unique industrial views.

Mysterious rocks that ring musically when struck.

With a history dating back to Victorian times, this architectural gem is a rare New York tourist haunt worth visiting.

A Hell's Kitchen synagogue that has counted stars of stage and screen among its congregation.

An aptly-named street near the birthplace of the modern LGBT rights movement.

Sip a cappuccino in the historic cafe that introduced them to the United States.

This castle-like building in Manhattan helped destigmatize cancer in the 19th century.

An unconventional space for art and science housed in a massive old warehouse.

The oldest continuously inhabited street in America.

Hiroshima survivor on Riverside Drive.

This was one of the first (and last) university-built and operated nuclear reactors.

One of the few remaining American elm trees in New York’s Tompkins Square Park was the birthplace of a new religion.

This enormous banquet-themed artwork honors over 1,000 notable women throughout history.

A Wisconsin kitsch palace from the 1930s lives its second life in 21st-century Brooklyn.

Discovered by cows, these upstate New York show caves are presented with a healthy dose of psychedelic folk art.

The library of every book lover's dreams.